How early should I arrive for my flight

You should check the airport website for security wait times to help you plan. As in the previous answer for domestic travel an hour and a half is a good rule of thumb, with a minimum of two hours when traveling internationally. SeaTac, at times, has long waits at security check points, and even the clear and pre-check are sometimes limited to a single point which can negate some of the speed benefit of the service.

It's a nonstop to OKC

Depends if it local flights east 11/2 hour. If it international definitely no less than 2 hours.. What makes it easy is Clear and pre check.